1. Field of the Invention
The invention concerns a method to detect a breathing movement of an examination subject corresponding to signal data by means of magnetic resonance, and a computer-readable storage medium to implement the method.
2. Description of the Prior Art
Magnetic resonance technology (in the following the abbreviation MR stands for magnetic resonance) is a known technique with which images of the inside of an examination subject (for example) can be generated. Expressed more simply, in an MR examination, consisting of one or more MR measurements, the examination subject is positioned in a comparatively strong, static, for the most part homogeneous basic magnetic field (field strengths from 0.2 Tesla to 7 Tesla or more) in an MR apparatus so that its nuclear spins (spins for short) orient along the basic magnetic field. The basic magnetic field is also termed B0 field. Radio-frequency excitation pulses are radiated into the examination subject to trigger nuclear magnetic resonances, the triggered nuclear magnetic resonances are measured and MR images (for example) are reconstructed based thereon. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored in a k-space matrix as complex numerical values. An associated MR image can be reconstructed by means of a multidimensional Fourier transformation from the k-space matrix populated with values.
In magnetic resonance imaging (“MRI”) the breathing movement of a patient to be examined by means of MR can lead to image artifacts known as ghosts (“ghosting”), and/or blurring and/or intensity loss in the generated images, primarily in an examination of the organs of the thorax and the abdomen, i.e., examination regions affected by the breathing movement of the patient. Additionally the breathing motion can lead to registration errors between generated images. These artifacts can hinder a finding on the basis of these images (for example by a physician) and can lead to the situation that lesions (for example) are overlooked.
Numerous techniques exist in order to reduce artifacts resulting from breathing movement. For example, respiratory triggering and respiratory gating are two classes of methods that are compatible with a plurality of imaging or spectroscopic sequences used in MR examinations (for example) in which the achievable resolution is not limited in principle (opposite to breath-holding) and that manage without, or at least with relatively little, patient cooperation.
Respiratory gating is an MR measurement during which the breathing of the patient is detected and associated with the acquired measurement data, and the repetition rate of the MR measurement (in particular its TR, thus the time between the successive excitation of a slice) is independent of the breathing of the patient. Rather, the repetition rate is controlled by a parameter or by an additional, different physiological signal, for example an EKG. The breathing information is then used (for example) to repeatedly acquire individually measured measurement data (packets)—that, for example, were acquired during stronger breathing movement—until they have been acquired in a more peaceful phase of the breathing. Another use of the breathing information can be to acquire k-space lines that are expected to be particularly movement-sensitive or k-space lines determining the image impression in an exceptional (quiet) breathing phase (what is known as “ROPE”—“respiratory ordered phase encoding”).
Respiratory triggering is a technique that synchronizes an MR measurement (an imaging MR measurement, for example) with the breathing of the freely breathing patient and attempts to acquire defined packets of measurement data only during a marked phase of the breathing cycle. The marked phase is for the most part the relatively quiet phase of the breathing cycle at the end of the expiration. The acquisition of the measurement data is thus triggered by the phase of the breathing cycle. If a specific slice is excited only once per trigger, the effective repetition rate (TR) of the measurement sequence is thus a whole-number multiple (v=1, . . . , k) of the mean breathing cycle of the patient.
The detection of the breathing of the patient as a physiological signal during the measurement is a requirement for respiratory triggering and respiratory gating. The detection of the breathing can occur with a pneumatic sensor, for example. Another known possibility is to detect the breathing of the patient with MR signals, known as navigator sequences. A navigator sequence is normally a short sequence that (for example) acquires MR signals of the diaphragm from which the diaphragm position of the patient at a point of the acquisition can be extracted. The navigator sequence is interleaved with an imaging sequence (for example) that acquires the measurement data for an image acquisition, which measurement data is desired for the MR examination. The diaphragm position determined with the use of the navigator sequences hereby supplies the input signal for a trigger or gating algorithm that is used.
Relative to external sensors such as pneumatic sensors to detect the breathing, such navigator sequences have economic advantages since no additional hardware is needed. Furthermore, the use of navigator sequences is normally perceived by users as simpler or less complicated in comparison to the use of an external breathing sensor (a pneumatic sensor, for instance). Such an external sensor requires an adjustment on the patient during the measurement preparation. Such an adjustment is not needed with navigator sequences.
Navigator sequences in connection with respiratory gating and respiratory triggering are widely used, for example in cardiac imaging. Here the navigator sequence normally acquires signals of the liver dome while the imaging sequence acquires signals of the heart.
The reason for positioning the sensitivity volume of the navigator sequence through the dome of the liver is the robust optimal detection of the diaphragm edge at that location due to the strong signal difference at the transition from liver to lung. However, a difficulty arises in modern systems known as “short bore systems”. These have a shorter z-FOV (FOV: “field of view”) than classical MR systems. The z-FOV is the extent of the spherical or cylindrical volume inside the primary magnet of the MR system in the axial direction, i.e. along the axis of the magnet, in which the basic magnetic field B0 has a specified homogeneity that is sufficient for imaging. “Short bore systems” thus have a z-FOV that is too short (for example in large patients and/or given a desired imaging in the lower abdomen or pelvis) to position both the sensitivity volume of the navigator sequence and the imaging slices within this homogeneity region FOV.
In a series of methods wherein the physiological breathing signal of a patient is derived from the movement of the diaphragm; a one-dimensional MR signal is normally acquired under a readout gradient oriented in the foot-head direction. The individual methods differ in the excitation, it conventionally being a goal of the excitation to not also excite static structures (for example the ribcage) since static structures overlap moving structures in the “navigator images” reconstructed from the one-dimensional navigator signal, which would hinder a detection of the diaphragm movement.
How such an excitation with a spin echo sequence can be achieved with an excitation slice tilted relative to the refocusing slice is described in the article “Spin-Echo M-Mode NMR Imaging” by Tetsuya Matsuda et al., appearing in the periodical “Magnetic Resonance Imaging” 27, 238-246 (1992). Only spins that are localized in the intersection surface of the two slices are thus refocused and form a spin echo that is detected under the readout gradient. A disadvantage of this method is the saturation of the magnetization in the two planes.
A second method described in “Rapid NMR Cardiography with a Half-Echo M-Mode Method” by C. Hardy, J. Pearlman, J. Moore, P. Roemer and H. Cline; J. Comput. Assist. Tomogr. 15, 868 (1991) uses what is known as a 2D RF pulse that has a cylindrical excitation profile in a gradient echo navigator sequence. This solves the saturation problem of the aforementioned method. However, this type of excitation is not particularly robust and therefore is at best conditionally suitable for clinical use.
In the prior art methods are also known that do not derive the physiological signal from the position of the diaphragm.
For example, in U.S. Pat. No. 4,761,613 an additional signal with constant phase encoding moment is acquired under every readout prephasing gradient of a spin echo sequence. These echoes, what are known as monitor echoes, are subsequently compared with a reference monitoring echo and from the comparison it is decided whether the imaging data acquired during the subsequent spin echo are used for image reconstruction or not.
In Proc. Intl. Soc. Mag. Reson. Med. 14 (2006), p 2977 a gradient echo sequence is described in which the navigator signal is acquired in a 20 μs-long time window between the slice refocusing gradient and the delayed, switched phase encoding gradient (or, respectively, readout prephasing gradient). During the navigator time window all gradients are off. The magnitude of the navigator signal from the component coil with maximum signal forms a physiological signal point. The series of signal points that is obtained by means of a series of such navigators is subsequently lowpass-filtered and used as an input signal for a respiratory gating algorithm. A disadvantage of this method is that the minimum echo time of the sequence is extended by the time separation of the slice refocusing and phase encoding gradients (or, respectively, the readout prephasing gradient). Furthermore, it is disadvantageous that a projection of the complete excitation volume is used as a navigator signal. This can lead to a dephasing of the signal given the presence of spatially inconstant B0 inhomogeneities. Furthermore, the method—like all so-called “self-gated” or “self-navigated” methods in which both navigator data and image data are acquired after one excitation pulse—is not compatible with respiratory triggering.
A similar evaluation of an acquired navigator signal via digital filtering is also described in U.S. Pat. No. 4,961,426. However, there either the navigator data are acquired under a second readout gradient after the acquisition of the image data and after a rephasing the phase encoding moment or image data and navigator data are acquired in an interleaved manner after a respective separate excitation.
How the movement of the ribcage can be extracted from projection data as a physiological signal correlated with the breathing is described in the article “Extraction of Cardiac and Respiratory Motion Cycles by Use of Projection Data and Its Applications to NMR Imaging” by W. S. Kim et al.; Magnetic Resonance in Medicine 13, 25-37 (1990). The projection data are acquired between excitation pulse and refocusing pulse of the imaging spin echo sequence.
Respiratory gating and respiratory triggering methods are used not only for imaging or spectroscopic MR examinations of the heart but also for MR examinations of the abdomen or pelvis, for example. However, in contrast to MR examinations of the heart, a detection of the breathing movement by means of navigator sequences in examinations of the abdomen and pelvis is disproportionately more difficult. One reason for this is that an excitation volume for the navigator sequences and an examination volume for the MR examination are both situated in the region of the abdomen and, in general, will overlap.
In order to reduce interference—for example saturation bands in anatomical images of a breath-triggered imaging MR examination—as a result of the navigator excitation that also acts on the examination volume, an excitation pulse of the navigator sequence may only generate a small flip angle of the excited spins so that only a small saturation enters the magnetization, for example. However, a signal that is generated with such a small flip angle has a poor signal-to-noise ratio. Furthermore, the small flip angle condition precludes spin echo techniques.
Not only can the navigator excitation can have an interfering influence on the excitation of the breath-triggered MR examination, but also the reverse may be true. For example, an evaluation of the generated navigator data can be hindered by influences of the breath-triggered MR examination. For example, the excitation of the imaging slices by an imaging sequence can generate saturation bands in the navigator image. The positions of these saturation bands may vary spatially (for example in an interleaved, multi-slice measurement) and the intensities of these saturations bands will decline over time. In the series of the navigator images in which the acquired navigator data are shown, the saturation bands are therefore structures that vary over time. These must be differentiated from those structures whose variation over time is a consequence of the breathing movement to be detected (for example structures of the diaphragm edge).
Furthermore, there is a need for methods in order to acquire data that map the breathing movement of a patient and that can be used for respiratory gating and respiratory-triggering techniques.